Fluorinated carbon is known by the formula (CF.sub.x).sub.n, where x is a number between 0 and 2 and n is an indefinite number greater than 2. Hereafter, (CF.sub.x).sub.n is abbreviated as CF.sub.x. Fluorinated carbon is prepared from the reaction of fluorine gas with a crystalline or amorphous carbon. Graphite is a highly crystalline form of carbon. Amorphous carbon, such as petroleum coke, coal coke, carbon black and activated carbon, has a low degree of crystallinity in its structure. However, the degree of crystallinity in amorphous carbon can be increased by heat treatment at high temperatures.
The lattice constant (d.sub.002), representing the interlayer spacing between the two carbon layers, is a characteristic parameter for the degree of crystallinity of the carbon. A lattice constant (d.sub.002) is determined by the x-ray diffraction method using the K.alpha. line of copper (Cu) and calculated from Bragg's equation: ##EQU1## where .lambda. is the wavelength of the K.alpha. line, .eta. is a positive integer and, .theta. is the diffraction angle.
In the prior art, for example, U.S. Pat. No. 4,271,242 and the publication Nikon Kagaku Zasshi (1974, No. 6, page 1033), it is indicated that the lattice constant (d.sub.002) decrease with increase of heat treatment temperature. In other words, the degree of crystallinity of carbon increases with the increase of heat treatment temperature and the decrease of the lattice constant (d.sub.002). The commercially available coke has a lattice constant (d.sub.002) between 3.44 and 3.49.ANG.. When such a coke is heat treated at a temperature above 2,000.degree. C., its lattice constant (d.sub.002) decreases to 3.39 or less. When heat treatment temperature reaches 3,000.degree. C. or more, the coke is considered to be completely graphitized. A pure graphite has a lattice constant (d.sub.002) of 3.354.ANG..
One of the major applications of fluorinated carbon is in its use as a positive electrode in combination with a lithium or other active metal to give a battery of high energy density. For such applications, fluorinated carbon which is derived from amorphous carbon such as coke is preferred for use because it can be prepared in relatively high yields at relatively low temperatures and this is more economical than fluorinated carbon which is prepared from graphite which requires higher temperatures to obtain relatively high fluorine content. Thus, fluorination reaction temperatures used for amorphous carbon are, to my best knowledge and belief kept below about 450.degree. C. (See U.S. Pat. No. 4,271,242). Although amorphous carbon may have been fluorinated at temperatures above 450.degree. C. to the best of my knowledge, no such fluorinated carbon material has been sold or publicly used in a metal/CF.sub.x battery.
For the purpose of this invention, amorphous carbon will be defined as carbon with a lattice constant (d.sub.002) of 3.37.ANG. or greater. This is intended to distinguish over pure graphite which, as indicated above, has a lattice constant of 3.354.ANG..
In the aforementioned U.S. Pat. No. 4,271,242, fluorinated carbon obtained by fluorinating amorphous or heat treated amorphous carbon contains a large amount of adsorbed fluorine which causes high initial voltage and poor shelf life in the non-aqueous Li/CF.sub.x battery. See, for example, column 6, line 50 to columns 7 and 8 of that patent. The batteries composed of fluorinated carbon made from high temperature heat treated coke show low capacity and small utility factor before and after storage as shown in Tables II and III in the aforementioned U.S. Pat. No. 4,271,242.
A need exists for an improved fluorinated carbon made from amorphous carbon or heat treated amorphous carbon to be free of the above drawbacks.